This invention intends to overcome a limitation of the wet process, which is commonly used to manufacture the ceramics parts of shape complexity. Such limitation arises when raw material powders have a wide range of particle sizes and density distribution for intended use of ceramics parts, which tend to promote particle separation in the wet process.
In general, using fine powders can improve the sinterability of ceramics parts. However, when these parts need to be used in a high temperature environment (e.g. sintering jig, reaction tube etc.), dispersed particles of coarse size are used instead in order to improve their creep resistance and thermal shock resistance. To achieve a high molding density using the powder of coarse size, pores among coarse particles are filled by intermediate size particles. Subsequently, smaller particles are then used to fill smaller pores formed by these intermediate size particles.
The critical factor to minimize pore volume and increase overall packing density is to maintain packing uniformity without segregation between component particles using the powders of multimodal particle size distribution.
As recent demands have increased for manufacturing large size ceramics parts in real form and improving their physical properties, use of the wet process such as slip casting, gel-casting and injection molding has increased gradually. However, the wet process is rarely used for molding powders of multimodal particle size distribution. This is because their sedimentation velocities vary greatly depending on different particle size and density under atmospheric gravity.
Generally, the larger the particle size ratio in constitutional particles, the higher the packing density by multimodal particle size distribution. However, it is reported that the best packing density can be achieved at the 7:1 size ratio and no appreciable benefit is gained by increasing the ratio any further (Particle Packing Characteristics, R. M. German, pp. 147, Metal Powder Industries Federation, New Jersey).
When the size ratio of constitutional particles is 7:1, their sedimentation velocity ratio becomes 49:1 according to Stokes law. Therefore, the particles tend to separate as illustrated in FIG. 1. To prevent such undesirable effects by maximizing the volumetric fraction of powders in the slurry, a significant improvement in packing homogeneity can be achieved by a restricted sedimentation as shown in FIG. 2 (G. A. Steinlage, R. K. Roeder, K. P. Trumble, and K. J. Bowman, “Centrifugal Slip Casting of Components,” Am. Ceram. Soc. Bull., 75(5): 92-94, 1996).
However, when trying to manufacture green bodies with high packing density using mixed powders of multimodal particle size distribution, it is very difficult to completely overcome particle separation in the wet process (e.g. centrifugal casting) as the liquid medium is not removed. This makes manufacture of homogeneous green bodies difficult.